CHAPTER XII 

 THE MAKING OF THE LIVING MATTER 



155. SOURCES OF FOOD. The ordinary green plant has 

 but two sources from which to obtain food,^the air and the 

 soil. When a plant is thoroughly dried in an oven, the 

 water passes off: this water came from the soil (154). 

 The remaining part is called the dry substance or dry 

 matter. If the dry matter is burned in an ordinary fire, 

 only the ash remains : this ash came from the soil (152). 

 The part which passed off as gas in the burning contained 

 the elements which came from the air: it also contained 

 some of those which came from the soil all those (as 

 nitrogen, hydrogen, chlorine) which are transformed into 

 gases by the heat of a common fire. 



156. CARBON. Carbon enters abundantly into the com- 

 position of all plants. Note what happens when a plant 

 is burned without free access of air, or smothered, as in a 

 charcoal pit. A mass of charcoal remains, almost as large 

 as the body of the plant. Charcoal is almost pure carbon, 

 the ash present being so small in proportion to the large 

 amount of carbon that we look on the ash as an im- 

 purity. Half or more of the dry substance of a tree 

 is carbon. When the tree is charred (or incompletely 

 burned), the carbon remains in the form of charcoal. The 

 carbon goes off as a gas when the plant is burned in air. 

 It does not go off alone, but in combination with oxygen, 

 and in the form called carbon dioxid gas, CO2. 



157. The green plant secures its carbon from the air. 

 In other words, much of the solid matter of the plant 

 comes from one of the gases. By volume carbon dioxid 



(74) 



